Rod guides for centralizing sucker rods within production tubing are well known in the art. As shown in FIG. 1, a pumping unit has attached thereto a sucker rod 10. (FIG. 1 was copied from U.S. Pat. No. 5,180,289 to Wenholz et al. and assigned to Baker Hughes Incorporated). At the bottom end of the sucker rod 10 is a reciprocating pump (not shown). As the pumping unit moves the sucker rod 10 down, the barrel of the reciprocating pump fills with the production fluid to be produced. Conversely, as the pumping unit moves the sucker rod up, a valve in the reciprocating pump shuts and the production fluid in the pump barrel is lifted, displacing production fluid above it and forcing one pump-barrel's worth of production fluid out of the hole.
The sucker rod must extend from the pumping unit all the way down to the reciprocating pump, which may be several thousand feet below the surface. Consequently, the sucker rod is subjected to a variety of stresses: compression, tension, torsion, and bending. Further, the sucker rod can "wobble" or bend within the production tubing. This problem of "wobble" or bending has been solved by the installation of rod guides on the sucker rod to centralize the sucker rod within the production tubing thereby controlling rod and tubing wear.
In carrying out this function, the rod guide, generally made of a polymeric material, is subjected to wear. As the rod guide wears down, couplings connecting the various rod guide segments together may come into contact with the tubing. When viewing a partially worn prior art rod guide a certain amount of guesswork is involved in determining the remaining useful life of the rod guide before the couplings begin to suffer wear themselves. Thus, there remains a need for a rod guide that provides a clear, easily determinable gauge of rod guide vane wear.
A prior art sucker rod guide includes a body that is molded in intimate contact with the sucker rod. The body has simultaneously molded therewith a plurality of "fins" or "blades" that extend radially from the body. As used herein, the term "fin" or "blade" refers to the molded portion of the rod guide that extends from the body to guidingly contact the interior surface of production tubing.
As noted above, rod guides are subject to a variety of stresses. One such stress on rod guides results from a bending moment that has been shown to be one significant source of rod guide failure. One reason for this is that rod guides are primarily made of plastic that is molded directly upon a sucker rod. Rod guides are commonly made from a material that conforms to a standard from the National Association of Corrosion Engineers (NACE), Std. TM-01-87-Hydrocarbon Mixture With 500 psi gas consisting of 87.5% CO.sub.2 and 12.5% H.sub.2 S. This standard dictates a material which is resistant to temperature and chemicals (e.g., H.sub.2 S, certain salts, etc.) and such a material is inherently brittle. Rod guides are commonly made of ryton, nylon, polyurethane, or the like.
To provide a predictable site for rod guide failure, Positive Action Tool Co. of Dallas action produced a rod guide known as "double-plus." "Double-plus" provided two pairs of fins, offset circumferentially from one another by 90.degree.. However, such an arrangement apparently does nothing to reduce the likelihood of such a failure, it simply predetermines where such a failure will occur. Also, such a design presents the same resistance to fluid flow and, in fact, appears to make undesirable turbulent flow more likely.
Thus, there remains a need for a rod guide that is more robust to bending moment without sacrificing any of the other important features previously noted.